Over the past decade, storage densities in magnetic recording technology haves experienced a tremendous increased in storage densities with a corresponding decrease in and the cost per GByte has correspondingly decreased. However, due to limitations imposed by the super-paramagnetic effect, the maximum achievable storage capacity of magnetic recording technologies is believed to be in the order of 250 Gbit/in2.
Another shortcoming of hard disk drives is their high access time. The large physical size of hard disk drives is a major contributor to this high access time. MEMS-based scanning-probe data storage technology is considered as an ultra-high-density and low-access-time alternative to conventional magnetic data storage that addresses some of the shortcomings of the latter. Such a probe-storage device uses nanometer-sharp tips, typically used in scanning probe microscopes to write information to and read the recorded information from a storage medium. Such a probe-storage concept is, for example, disclosed in H. J. Mamin, R. P. Ried, B. D. Terris, and D. Rugar, “High-Density Data Storage Based On The Atomic Force Microscope,” Proc. IEEE, vol. 87, pp. 1014-1027, 1999. Such a technology can be regarded as a candidate to previously-proposed technologies for achieving higher storage densities at lower access times. A major challenge in creating a viable probe-storage device is that such probes operate for read and write operations, respectively, at best on the microsecond timescale. Therefore, in order to be competitive with known conventional storage technologies, orders of magnitude increases in speed are necessary is desirable for such a device. One solution to achieve such a substantial increase in data rates is to employ with MEMS-based arrays of probes operating in parallel, with each probe performing read/write/erase operations on an individual storage field. In such a system, a MEMS-based micro-scanner with two-dimensional motion capabilities is used to position the storage medium with respect to the array of read/write probes. Since actuation distances typically are of the order of 100 μm and the moving components have relatively smaller masses, the access delays are generally expected to be smaller than in reduced compared to disk drives.
Implementations of probe-based storage devices are known for example from P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. D{umlaut over ( )}uürig, B. Gotsmann, W. Häberle, M. Lantz, H. Rothuizen, R. Stutz, and G. Binnig, “The “Millipede”—Nanotechnology Entering Data Storage,” IEEE Transactions on Nanotechnology, Vol. 1, pp. 39-55, 2002 and E. Eleftheriou, T. Antonakopoulos, G. K. Binnig, G. Cherubini, M. Despont, A. Dholakia, U. D{umlaut over ( )}urig, M. A. Lantz, H. Pozidis, H. E. Rothuizen, and P. Vettiger, “Millipede—A MEMS Based Scanning-Probe Data Storage System,” IEEE Transactions On Magnetics, vol. 39(2), pp. 938-945, 2003.
Such devices are based on a thermo-mechanical principle for storing and retrieving information written on thin polymer films. Thereby, such films typically have a thickness less than 200 nm. Digital information is stored by making indentations on the thin polymer film by using the tips of AFM micro-cantilevers, which are a few nanometers in diameter. The shape of a typical indentation resembles an almost conical structure with a diameter of approximately 15 to 30 nm. This indentation shape results in an error-rate performance that increasingly rapidly deteriorates as the probe-tip distance from the center of the indentation is increased. This is, for example, discussed H. Pozidis, W. Häberle, D. W. Wiesmann, U. Drechsler, M. Despont, T. Albrecht, and E. Eleftheriou, “Demonstration of Thermomechanical Recording at 641 Gbit/in2,” IEEE Transactions On Magnetics, vol. 40(4), pp. 2531-2536, 2004.
Hence accurate positioning of the storage medium relative to the probes is essential. Typically a micro-scanner is employed to position the storage medium. Such a micro-scanner is disclosed in M. A. Lantz, H. Rothuizen, U. Drechsler, W. Haeberle and M. Despont, “A vibration resistant Nanopositioner For Mobile Parallel-Probe Storage Applications”, Journal of Microelectromechanical Systems, 2006.
For controlling the micro-scanner in a closed-loop fashion, position information is derived from a global position sensor like a thermal position sensor that is disclosed in M. A. Lantz, G. K. Binnig, M. Despont, and U. Drechsler, “A Micromechanical Thermal Displacement Sensor With Nanometer Resolution,” Nanotechnology, Vol. 16, pp. 1089-1094, May 2005.
Track seeking and track following controllers may, for example, utilize thermal sensors. For example A. Pantazi, A. Sebastian, G. Cherubini, M. Lantz, H. Rothuizen, H. Pozidis, and E. Eleftheriou, “Control of Mems-based Probe Storage Devices,” IEEE Transactions on Control System Technology, 2006, discloses a prototype system that uses thermal position sensors for deriving positional information.
Thermal sensors have a satisfactory noise performance at increased high frequencies. However, they tend to suffer from significant low-frequency drifts which can be considered as low-frequency noise. Hence a feedback-control scheme relying on the thermal sensors alone is not suitable for a long-term operation of the device. Because of the availability of multiple probes, a small number of probes and their respective storage fields could be dedicated for the generation of some form of medium-derived positional error signal (PES). This medium-derived PES accurately captures deviations from the track centerline for each data track. Therefore, it has a very limited range that is substantially equal to the distance between tracks.
Nevertheless, since medium-derived PES is crucial for a satisfactory operation of such a micro-scanner, prior to using a MEMS-based scanning probe storage device, the servo information generating the medium-derived PES has to be written in those storage fields reserved for this specific purpose. This operation is referred to as “servo writing”. Since the servo information is written without assistance from an external positioning device, the servo writing is usually called “self servo writing”.
An object of the present invention is to provide an improved method of controlling movements of a positioner of a scanner, an improved controller, an improved scanner, an improved data storage device and an improved scanning probe microscope.